multi: move BackupState and RegisterChannel to Manager

This commit moves over the last two methods, `RegisterChannel` and
`BackupState` from the `Client` to the `Manager` interface. With this
change, we no longer need to pass around the individual clients around
and now only need to pass the manager around.

To do this change, all the goroutines that handle channel closes,
closable sessions needed to be moved to the Manager and so a large part
of this commit is just moving this code from the TowerClient to the
Manager.

server.go

@@ -284,10 +284,6 @@ type server struct {
 
 	towerClientMgr *wtclient.Manager
 
-	towerClient wtclient.Client
-
-	anchorTowerClient wtclient.Client
-
 	connMgr *connmgr.ConnManager
 
 	sigPool *lnwallet.SigPool
@@ -1577,7 +1573,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		}
 
 		// Register a legacy tower client.
-		s.towerClient, err = s.towerClientMgr.NewClient(policy)
+		_, err = s.towerClientMgr.NewClient(policy)
 		if err != nil {
 			return nil, err
 		}
@@ -1589,9 +1585,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 			blob.Type(blob.FlagAnchorChannel)
 
 		// Register an anchors tower client.
-		s.anchorTowerClient, err = s.towerClientMgr.NewClient(
-			anchorPolicy,
-		)
+		_, err = s.towerClientMgr.NewClient(anchorPolicy)
 		if err != nil {
 			return nil, err
 		}
@@ -3782,6 +3776,17 @@ func (s *server) peerConnected(conn net.Conn, connReq *connmgr.ConnReq,
 		}
 	}
 
+	// If we directly set the peer.Config TowerClient member to the
+	// s.towerClientMgr then in the case that the s.towerClientMgr is nil,
+	// the peer.Config's TowerClient member will not evaluate to nil even
+	// though the underlying value is nil. To avoid this gotcha which can
+	// cause a panic, we need to explicitly pass nil to the peer.Config's
+	// TowerClient if needed.
+	var towerClient wtclient.TowerClientManager
+	if s.towerClientMgr != nil {
+		towerClient = s.towerClientMgr
+	}
+
 	// Now that we've established a connection, create a peer, and it to the
 	// set of currently active peers. Configure the peer with the incoming
 	// and outgoing broadcast deltas to prevent htlcs from being accepted or
@@ -3820,8 +3825,7 @@ func (s *server) peerConnected(conn net.Conn, connReq *connmgr.ConnReq,
 		Invoices:                s.invoices,
 		ChannelNotifier:         s.channelNotifier,
 		HtlcNotifier:            s.htlcNotifier,
-		TowerClient:             s.towerClient,
-		AnchorTowerClient:       s.anchorTowerClient,
+		TowerClient:             towerClient,
 		DisconnectPeer:          s.DisconnectPeer,
 		GenNodeAnnouncement: func(...netann.NodeAnnModifier) (
 			lnwire.NodeAnnouncement, error) {